Cellular phones are desired to be usable for a long period of time with a limited amount of battery capacity. A power amplification section for amplifying a high frequency signal and supplying transmission power to an antenna especially consumes a large amount of power. Therefore, it is important to improve the efficiency of the power amplification section.
Various communication systems have become widely used in the world, and a multi-mode wireless communication terminal which is usable with a plurality of communication systems is desired. However, a multi-mode wireless communication terminal produced based on the conventional technology inevitably includes a larger number of components, especially in the wireless section, and is increased in size and cost. In order to realize a multi-mode wireless communication terminal, it is an important object to commonly use components for a plurality of communication systems.
The EDGE (Enhanced Data GSM Environment) system has been developed for increasing the communication speed based on the GSM (Global System for Mobile Communications), which is one cellular phone standard. With the EDGE system, a polar modulation (polar modulation) system is often adopted as a modulation system in a wireless transmission section. The polar system is highly compatible with the transmission circuit configuration of the GMSK (Gauss filtered Minimum Shift Keying) modulation, which is a conventional modulation system.
With a multi-mode wireless communication terminal compatible with the GSM system and the UMTS (Universal Mobile Telecommunications System), adoption of the polar system results in a uniform platform and thus simplification of the system.
In order to restrict the size enlargement of the wireless section, it is effective to, for example, commonly use components such as a power amplifier and the like for a plurality of communication systems. However, such a step generally reduces the operation efficiency. In addition, the UMTS requires the power to be controlled over a range from a very low output to a high output. Therefore, it is also important to securely obtain a transmission dynamic range.
With the above-described background, the following technologies have been proposed.
Means for controlling a power supply section for polar modulation operation of the power amplification section is disclosed in the specification of U.S. Pat. No. 6,528,975 (hereinafter, referred to as “patent document 1”) FIG. 20 is a block diagram showing a structure of an amplitude control section 900 in a polar modulator described in patent document 1.
As shown in FIG. 20, the amplitude control section 900 includes an input terminal 901, a DAC (digital-analog converter) 902, a controller 903, a comparator 904, an OP amplifier 905, a current monitor 906, a power supply terminal 907, a load 908, a transistor 909, and a power supply terminal 910.
Amplitude data which is input from the input terminal 901 is input to the OP amplifier 905 via the DAC 902. The output voltage of the OP amplifier 905 is input to a base of the transistor 909 via the current monitor 906. Thus, the transistor 909 amplifies the signal which is input to the base based on a supply voltage Vbat which is input from the power supply terminal 910, and outputs the amplified signal as a modulated signal voltage Vm(t). The load 908 is a power amplifier. The transistor 909 modulates the power supply terminal 907 of the power amplifier connected thereto as the load 908 using the modulated signal voltage Vm(t). The modulated signal voltage Vm(t) is fed back to the OP amplifier 905. The OP amplifier 905 compares a modulated signal which is output from the DAC 902 and the modulated signal voltage Vm(t) which is fed back, and adjusts the output voltage.
A saturation voltage of 0.1 V is present between an emitter and a collector of the transistor 909. Therefore, the supply voltage Vbat of the transistor 909 needs to be higher by at least 0.1 V than the maximum value of the modulated signal voltage Vm(t). Otherwise, the transistor 909 is saturated and becomes inoperable. When the transistor 909 is saturated, an electric current flows into the base thereof. The current monitor 906 monitors the electric current flowing into the base, and transfers the monitoring result to the comparator 904. The comparator 904 determines whether or not the electric current monitored by the current monitor 906 is equal to or larger than a predetermined threshold value. When the electric current is equal to or larger than the predetermined threshold value, the comparator 904 transfers such information to the control section 903. In accordance with this, the control section 903 reduces the maximum value of the output voltage from the DAC 902 until the comparator 904 stops transferring the information that the current is equal to or larger than the predetermined threshold value via the current monitor 906. By reducing the maximum value of the output voltage from the DAC 902 as described above, the Vbat can be kept higher by at least 0.1 V than the maximum value of the modulated signal voltage Vm(t). Thus, the transistor 909 can be prevented from being saturated.
When the power supply terminal 910 of the transistor 909 is provided with a sufficiently high supply voltage Vbat, the above-described control is not necessary. However, the supply voltage Vbat is usually supplied from a battery. Accordingly, when the capacity of the battery is reduced, the above-described control becomes necessary. Namely, when the amplification control section 900 described in patent document 1 is used, a stable operation is realized without the transistor 909 being saturated even if the supply voltage Vbat is lowered due to the reduction in the voltage of the battery.
An example of enlarging the dynamic range in a polar modulator is disclosed in the specification of U.S. Pat. No. 6,242,975 (hereinafter, referred to as “patent document 2”). FIG. 21 is a block diagram showing a structure of a polar modulator 923 described in patent document 2.
As shown in FIG. 21, the polar modulator 923 includes an envelope extractor/regulator 911, a variable amplifier 912, a power amplifier 913, and a quadrature modulator 914. An I (Inphase) baseband signal and a Q (Quadrature) baseband signal are input to the envelope extractor/regulator 911. The envelope extractor/regulator 911 outputs an Ic signal and a Qc signal with which the amplitude component (I2+Q2)1/2 is constant, and also outputs an amplitude signal indicating an envelope component of the I and Q base band signals. The variable amplifier 912 regulates a component of the amplitude signal larger than a predetermined value to the predetermined value. The quadrature modulator 914 generates a phase modulated signal based on the Ic signal and the Qc signal which are input thereto, and outputs the phase modulated signal to the power amplifier 913. The amplitude signal is adjusted in terms of the amplitude by the variable amplifier 912, and then is input to a power supply section of the power amplifier 913. The power amplifier 913 modulates the amplitude of the phase modulated signal using the amplitude signal, and outputs a modulated signal. The envelope extractor/regulator 911 regulates the amplitude of the amplitude signal. By this operation, a peak to average power ratio (PAPR), which is the ratio of the maximum value with respect to the average value, of the modulated signal is reduced. As a result, the changing width of the amplitude signal is suppressed, and thus the dynamic range is enlarged.
In the case where the dynamic range is enlarged, the range in which the amplifying element used in the power amplification section has linearity is enlarged.
FIG. 22 is a graph illustrating the relationship between the adjacent channel leakage power (ACP) and the PAPR of an amplitude signal in an output from a cellular phone using the IS95 system. In the case where, for example, the transmission frequency is 1980 MHz, the ACP does not increase when the PAPR is equal to or larger than 10 dB. By contrast, when the PAPR is smaller than 4.2 dB, the ACP goes outside −54 dBc which is the specified value. Namely, as long as the PAPR of the amplitude signal is regulated to be equal to or larger than 4.2 dB, the ACP never goes outside the specified value. Therefore, the envelope extractor/regulator 911 regulates the amplitude of the amplitude signal so as not to go outside the specified value. In this way, an average output which is larger by 6 dB at the maximum is obtained than the case where the PAPR is not regulated as described above. As a result, the dynamic range is enlarged.
Another example of enlarging the dynamic range in a polar modulator is disclosed in the specification of the United States Laid-Open Patent Publication No. 2005/0008093 (hereinafter, referred to as “patent document 3”). FIG. 23 is a block diagram showing a structure of a polar modulator 922 described in patent document 3.
As shown in FIG. 23, the polar modulator 922 includes an amplitude signal input terminal 915, an amplitude regulation circuit 916, a voltage control circuit 917, a phase signal input terminal 918, an angle modulator 919, a power amplifier 920, and an output terminal 921.
The angle modulator 919 modulates the angle of a phase signal which is input thereto. When the amplitude of an input amplitude signal becomes smaller than a first value, the amplitude regulation circuit 916 and the voltage control circuit 917 shape the waveform of the amplitude signal, such that the amplitude of a portion of the amplitude signal which is smaller than the first value is increased to the first value. When the amplitude of the input amplitude signal becomes larger than a second value, which is larger than the first value, the amplitude regulation circuit 916 and the voltage control circuit 917 shape the waveform of the amplitude signal, such that the amplitude of a portion of the amplitude signal which is larger than the second value is decreased to the second value. The power amplifier 920 modulates the amplitude of the angle modulated wave which is output from the angle modulator 919 using a signal which is output from the voltage control circuit 917, and thus outputs an amplitude modulated voltage. By this operation, the maximum value of the amplitude signal which is input to the power amplifier 920 can be regulated. As a result, even if the amplifying element used in the power amplifier 920 does not have linearity in a wide range, a desirable signal having little distortion can be obtained. Thus, the dynamic range is enlarged. By contrast, when the structure shown in patent document 3 is adopted, a power amplifier having a smaller range of linearity but consuming a smaller amount of power can be used. In this case, the efficiency can be improved.
The technology described in patent document 1 has the following problem. With the structure described in patent document 1, when the supply voltage Vbat is lowered, the amplitude control section 900 operates with the maximum possible amplitude modulated voltage Vm(t) at which the transistor 909 is not saturated. Accordingly, when the supply voltage Vbat is lowered, a high operation efficiency is realized. However, when the supply voltage Vbat is apparently higher by at least 0.1 V than the maximum value of the amplitude modulated voltage Vm(t), for example, immediately after the battery is charged, the power using efficiency is reduced. As wireless communication is increased in speed and capacity, transmission circuits are desired to operate more and more efficiently. In a structure in which an amplitude signal is input from a series regulator provided with a fixed value of supply voltage to the power amplifier so as to perform polar modulation, it is most important to suppress the loss in the series regulator in order to realize a highly efficient operation. However, in the amplitude control section 900 of patent document 1, the efficiency is lowered when the supply voltage Vbat is high, which is not preferable.
The structures described in patent documents 2 and 3 can enlarge the dynamic range and increase the efficiency, but have the following problems. The technologies of patent documents 2 and 3 increase the efficiency by reducing the size of the power amplifier. However, when, for example, one polar modulator is commonly used for the GSM system and the UMTS, the size of the power amplifier cannot be reduced. The reason is that it is determined by the standards that the maximum output of the operation by the EDGE system or the UMTS is smaller than the maximum output of the operation by the GMSK system, and thus the size of the power amplifier is determined by the maximum output of the GMSK modulation operation.
The specified value of the ACP varies in accordance with the modulation system used. The technologies described in patent documents 2 and 3 control the PAPR and thus enlarge the dynamic range by controlling the amplitude of the amplitude signal to be uniform. However, when a plurality of modulation systems are usable, such uniformization of the amplitude of the amplitude signal may result in the dynamic range being not securely obtained. By contrast, when a regulated value of the amplitude signal is increased in order to securely obtain the dynamic range, the efficiency may be reduced or the signal may be distorted.
As described above, the conventional technologies are not sufficient in increasing the power utilization efficiency and/or enlarging the dynamic range. Especially when a plurality of modulation systems are usable, for example, in a multi-mode operation, it is difficult to increase the power utilization efficiency and/or to enlarge the dynamic range with the conventional technologies.
Accordingly, an object of the present invention is to provide a polar modulator which can increase the power utilization efficiency and/or enlarge the dynamic range, and a wireless communication apparatus using the same.